We have continued our studies of chromatin structure, in order to understand the forces that are involved in the packaging of DNA within eukaryotic nuclei, and to determine how that structure is modified in the neighborhood of genes that are being expressed. We have extended studies of the phasing of nucleosomes on DNA, and have found that for at least one arbitrarily chosen 145-base-pair DNA fragment, the placement of the histone octamer with respect to the ends of the DNA is asymmetric, showing that there are strong sequence-dependent forces of interaction between protein and DNA. In experiments concerned with the structure of chromatin containing transcriptionally active genes, we have examined the physical properties of nucleosome-like particles containing the chicken adult Beta globin gene, isolated from erythrocytes in which that gene is being expressed. We find that these particles possess properties very similar to those of normal nucleosome core particles, showing that the packaging of active genes at this level of organization is similar to that of bulk chromatin. We have also continued studies of the structure of DNA in the promoter region of this Beta globin gene, which we have found has s special chromatin structure when the gene is expressed, as demonstrated by an unusual sensitivity to nucleases. We have been able to reconstruct this sensitive structure by assembling chromatin on globin-gene containing plasmids in the presence of extracts from chicken eythrocyte nuclei. We have shown that these extracts contain one or more factors that bind specifically and tightly to the promoter region, and that appear to be responsible for generating the unusual chromatin structure of the region. The factor has been partially purified.